Testing motor grounding and somatotopic effects for literal and figurative action-language in Motor Neuron Diseases: a multiple-case analysis

Motor Neuron Diseases (MNDs) represent a test-ground to assess motor cortex involvement in action-language from literal to figurative uses, also taking into account somatotopicity. Here, a sample of patients with Amyotrophic Lateral Sclerosis (ALS) and another with the SPG4 variant of Hereditary Spastic Paraplegia (HSP-SPG4), affecting specifically lower limbs, were tested with two tasks employing upper-limb and lower-limb motion verbs either as literal isolated words or in literal and metaphorical sentences, controlling for cognitive impairment. While in ALS literal action-language impairment was negligible, in HSP-SPG4 it affected up to 50% of the patients with the expected disadvantage for lower-limb verbs. Figurative action language was impaired in 20% of ALS and 40% of HSPG4 patients, but somatotopic effects were limited. Findings support the motor simulation account at literal and figurative levels. Yet they also highlight the gradual nature of motor grounding disruption, depending on linguistic contexts, clinical profiles, and reorganization mechanisms.